The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
CA carrier aggregation
CC component carrier
CIF carrier indication field
C-RNTI cell RNTI
DL downlink
eNB node B/base station in an E-UTRAN system
E-UTRAN evolved UTRAN (LTE)
LTE long term evolution
LTE-A long term evolution-advanced
MAC medium access control
PCC/PCell primary component carrier/primary cell
PDCCH physical downlink control channel
PDU protocol data unit
P-RNTI paging RNTI
PUCCH physical uplink control channel
PUSCH physical uplink shared channel
RACH random access channel
RA-RNTI random access RNTI
RAR random access response
RNTI radio network temporary identifier
SCC/SCell secondary component carrier/secondary cell
SI-RNTI system information RNTI
TA timing advance
UE user equipment
UL uplink
The bandwidth extension beyond 20 MHz in LTE-A (expected to be implemented in 3GPP Release 11) is done via carrier aggregation CA, in which multiple component carriers CCs are aggregated together to form a larger bandwidth. This is shown by example at FIG. 1A in which there are five Release 8 compatible CCs aggregated to form one larger LTE-A bandwidth. There is at least one CC which is backward compatible with legacy (3GPP Release 8/9) user terminals, 20 MHz wide and having all the control and traffic channel structure of Release 8. FIG. 1A is exemplary; in practice there may be more or less than five CCs, they may not have equal bandwidths, they may be frequency non-adjacent, and LTE-A is considering the case where one or more secondary CCs are in unlicensed spectrum. The CCs could be aggregated in both time division duplex TDD and frequency division duplex FDD systems.
FIG. 1A-B illustrate different exemplary scenarios in which CA may be employed. At FIG. 1B there is a macro cell F1 (e.g., a traditional cellular base station) which provides macro-area coverage and further there are remote radio heads (RRHs) F2, controlled by the macro cell F1, which are used to improve throughput at hot spots (hot spots shown by the darker shading at FIG. 1B). Mobility is performed based on the F1 coverage (shown by lighter shading at FIG. 1B). By example, F1 and F2 may operate on different bands, e.g., F1={800 MHz, 2 GHz} and F2={3.5 GHz}, etc. It is expected that F2 RRHs cells can be aggregated with the underlying F1 macro cells. In this case a UE would communicate with the F1 cell on one CC (typically the PCC) and with the F2 cells on different CCs (SCCs). These different CCs are seen by the UE as different cells.
FIG. 1C illustrates a different CA scenario, similar to that of FIG. 1B but in which frequency selective repeaters are deployed so that coverage is extended for one of the carrier frequencies. It is expected that F1 and F2 cells of the same eNB can be aggregated where coverage overlaps. In this case also the UE will see the different CCs as different cells where the F1 and F2 cells are operating on different frequencies/different CCs.
In LTE Release 8, the PDCCH which informs the individual UEs which radio resources are allocated for their traffic could only be used to indicate a PDSCH/PUSCH sent on its own DL CC or its paired UL CC (since using the hindsight of CA the Release 8 spectrum can be considered as only one CC). In LTE-Advanced, “cross-scheduling” can be available, which means the PDCCH could be used to indicate PDSCH/PUSCH resources sent on other CCs other than its own DL CC and/or its paired UL CC. From the perspective of the transmitted PDCCH this cross-scheduling is useful for distributing traffic loads among the multiple carriers.
But in the scenarios of FIGS. 1B-C the transmitters are at different distances from the UE, and so there are propagation delays to be compensated. In LTE the eNB signals a timing advance (TA) to the UE as detailed at 3GPP TS 36.321v10.1.0 (2011-March) at section 5.2. When receiving a timing advance command (TAC), the UE adjusts its uplink transmission timing as detailed at 3GPP TS 36.213 v10.0.1 (2010-December) section 6.1.1. A timing advance command can be received in a random access response or in a MAC control element. The validity of a timing advance command is controlled by the TA timer in the UE. As long as the TA timer is running, the timing advance remains valid and uplink transmissions can take place on the shared channel. Every time a timing advance command is received, the TA timer is restarted. When the TA timer expires, uplink synchronization is required and no uplink transmission can take place on the shared channel. In order for the eNB to assess the timing adjustment needed at the UE, a random access procedure is usually started.
In the continuing development of 3GPP Release 11 a new CA work item was described to “specify the support of the use of multiple timing advances in case of LTE uplink carrier aggregation” (see document RP-110451 at section 4 “Objective”; 3GPP TSG RAN Meeting #51; Kansas City, USA; Mar. 15-18, 2011). Multiple TAs are needed to cope with network-side receivers which are not co-located, such as the RRH and frequency selective repeater scenarios illustrated at FIGS. 1B-C.
3GPP Release 10 specifies that cross carrier scheduling may be used to schedule resources on a cell from another cell. The carrier indicator field (CIF) allows the PDCCH of a serving cell addressed to a UE's C-RNTI to identify another cell in which the scheduled resources lie, but 3GPP 36.300 v10.3.0 v (2011-March) at section 11.1 sets forth the following restrictions:                Cross-carrier scheduling does not apply to PCell i.e. PCell is always scheduled via its PDCCH;        When the PDCCH of an SCell is configured, cross-carrier scheduling does not apply to this SCell i.e. it is always scheduled via its PDCCH;        When the PDCCH of an SCell is not configured, cross-carrier scheduling applies and this SCell is always scheduled via the PDCCH of one other serving cell.        
In 3GPP Release 10, no cross carrier scheduling is specified for all RACH related steps, as RACH is only supported on the PCell for Release 10 and the PCell cannot be scheduled from an SCell. When CA is configured, each CA-capable UE is configured with one PCell and optionally one or more SCells as its Serving Cell(s) but regardless the UE will have only one RRC connection with the network. The PCell is the one which provides the UE with its network access stratum mobility information, which is done at radio resource control (RRC) connection establishment, re-establishment or handover. The PCell is the one the UE uses for PUCCH transmissions, and unlike SCells once established the PCell can be changed only with a handover procedure and cannot be de-activated. So radio link failure on a PCell triggers a UE's re-establishment procedures, which is not the case if the failure is on the SCell. For further detail on the PCell and SCell(s) see 3GPP TS 36.300 v10.3.0 (2011-March) at section 7.5.
Cross scheduling scenarios such as those in FIGS. 1B-C are independent of the fact that multiple TAs might be needed. For example, assume a UE has its PCell with a macro cell F1 and also has a configured SCell with a RRH F2. If the UE loses its TA with the PCell it will perform a RACH on the PCell to obtain UL synchronization, but there is no possibility for the UL resources granted in that RACH procedure on the PCell to lie in the SCell. The possibility to cross schedule radio resources during a RACH procedure would give the network added flexibility Exemplary embodiments of the invention detailed below enable cross scheduling during a RACH procedure, which currently is not possible in LTE or LTE-A.
Besides, in 3GPP Release 10, the UE would not perform blind decoding in common search space of the SCells, as it is not expected to receive system information (scheduled by PDCCH addressed to SI-RNTI), nor paging (scheduled by PDCCH addressed to P-RNTI), nor random access response (scheduled by PDCCH addressed to RA-RNTI) on SCells. If supporting RACH procedure on SCell and the RAR of the SCell is scheduled from SCells, it would increase the number of blind decoding the UE needs to support. From this perspective, even if cross scheduling of the SCell is not configured, it would be beneficial if the RAR can be signaled from the PCell so that the UE only needs to decode the common search space of the PCell.